You can enable ssh access by editing the configuration file cmdline.txt on the LIBREELEC partition of the SD card, the one which is mounted under /flash into a running LibreELEC system. Just add the ssh kernel option:
boot=UUID=0403-3813 disk=UUID=35daecf4-1639-4f18-b69b-ce0498e092ca ssh quiet
The default SSH password for root is libreelec and cannot be changed. If you need to access the host using SSH and you want to secure it, just disable SSH passwords and use certificates.
Copy your public RSA key to the LibreELEC device and change the SSH configuration. The SSH daemon configuration is /storage/.cache/services/sshd.conf, set it to:
SSHD_DISABLE_PW_AUTH="true" SSH_ARGS="-o 'PasswordAuthentication no'"
Restart the daemon with systemctl restart sshd.service.
The daemon is running, edit your crontab with crontab -e.